Thermoplastic films are conventionally bonded by heat applied in a number of ways including hot knives and hot wires pressed into or passed through the films, ultrasonically vibrated heads pressed against the films and, more recently, radiant energy from resistance wires heated to incandescence by electrical energy. Hot knives and wires which touch the films have a tendency to cause burning of the plastic which emits noxious fumes and can cause undesirable puckering of the weld. Ultrasonic devices tend to be expensive, the quality of the seal is subject to variations, and the speed of operation can be slow. Radiant energy devices have the advantage of low energy consumption and, under the best conditions, improved seals and cuts because of the absence of physical contact with the plastic films. Heretofore, such devices have been subject to control problems in which the plastic films can be either overheated or underheated, causing imperfect seals as well as problems of uniform heating along the length of a seal due to changes in the lengths of the heated wires between their hot and cold modes. Accordingly, the present invention has for its objects to provide new and improved means for effecting the melting and sealing of thermoplastic films by radiant energy applied from a source which at no time contacts the films.